The present invention relates to a process for recovering solvents contained in a moisture laden exhaust gas stream exhausted from a paint spraying system. More specifically, it relates to such a process of solvent recovery used in combination with a continuous wet paint spray system using a complex mixture of solvents as a carrier for the coating or paint solids whereby this mixture of solvents is recovered in substantially the same ratio of components as previously used and is substantially free of water. The process operates at gas flow rates of 500 to 50,000 scfm with the exhaust gas stream entering the recovery process being 90% saturated with water to reduce the solvent vented from the system to less than about 200 ppm and, preferably, 100 ppm.
Coating or painting systems generally provide a means to apply a resinous coating material, referred to as coating solids, onto a workpiece. Wet coating systems apply the coating solids by dissolving the solids in a solvent or mixture of solvents and applying the formed solution onto the workpiece, e.g., by spraying the solution onto a workpiece from one or more spray nozzles. The solvent is then evaporated from the workpiece leaving a dry film of solid coating material adhering to the surface of the workpiece. The nature of many of the modern coatings or paints requires many different components and, in particular, a range of different solvents. These components are added in predetermined amounts by the paint formulator, and the proper components in the proper ratio must be present for the paint to perform optimally. Furthermore, with certain coatings, the solvent solutions must not contain water if they are to function properly. These solvents are also relatively expensive considering the quantity used.
One of the most serious problems encountered with spray coating systems and, particularly, with continuous coating systems is that of containing the material overspray and solvent vapors within the spray cabinet or booth in which the workpiece is coated so that they do not escape to the atmosphere. In a continuous coater which has open entrance and exit ports through which the continuously moving line of pieces to be coated moves, these ports are never closed during the spray cycle so that it is difficult to prevent airborne spray material and/or solvent vapors from escaping through these ports. The problem is compounded by the fact that the moving workpieces create air currents which tend to drag airborne overspray and solvent vapors which are heavier than air out of the coater cabinet. Moreover, fresh air tends to cling to the parts as they move into the coater and is "dragged in" with the parts lowering the internal solvent humidity.
A method to prevent material overspray and solvent vapors from escaping to the atmosphere through the entrance and exit ports is to add vestibules to the entrance and exit ports of the continuous coater which extend outwardly both from the entrance port and the exit port. These vestibules are connected to an exhaust system including an exhaust fan which is operable to pull outside air into the vestibule in sufficient quantities and at a sufficient velocity to effectively form a barrier to the egress of oversprayed material from the vestibules. To avoid emitting airborne paint solids through the exhaust to the atmosphere, water scrubbers have been employed in which the paint solids particles in the exhaust are trapped in a water spray and collected for disposal or recycle processing reuse. In a continuous coating system, the exhaust may be a gaseous stream flowing at a rate of less than about 50,000 scfm, e.g., 2500 scfm. This exhaust comprises ambient air drawn from the outside, vapor of the solvent used as a carrier for the paint solids at a rate of 4 to 15 gallons per hour (700-2625 ppm solvent vapor per hour) and water vapor at approximately 90% of saturation. The water vapor in the exhaust stream which exceeds ambient levels is present due to the water used in the scrubbers and is a by-product of the water scrubbing system.
In prior art systems of various types where organic vapors are present, these vapors are typically displaced to the atmosphere at 2500 ppm which is the safe flammable level under existing fire codes. However, in typical continuous coater systems as described above, the exhaust gas contains less than about 2500 ppm solvent vapor as it leaves the water scrubber. It is undesirable to exhaust the 2500 ppm of solvent in the exhaust gas into the atmosphere due to environmental concerns. Furthermore for economical reasons, it is highly desirable to collect and reuse the solvent.
Two problems are encountered in attempting to recover and reuse the solvent vapor as it exits the water scrubbers. First, the exhaust gas stream is highly saturated with water vapor, frequently up to 90% of saturation or more. Many solvents used in paint applications are extremely water sensitive and the presence of substantial amounts of water fouls the paint spray system. Second, the high degree of water saturation also prevents the use of certain known solvent recovery methods such as refrigeration methods since the water tends to freeze forming excessively high amounts of ice which fouls the recovery system and carbon beds since the presence of water reduces the available surface contact area drastically reducing the efficiency of the beds. To operate a carbon bed above about 80% relative humidity requires warming of the exhaust gas stream which lowers the overall efficiency of the process.
In addition, paint performance is extremely sensitive to the ratio of solvents in the paint and, thus, the solvent mixture which is recovered desirably should have the same ratio of component solvents as the solvent mixture originally employed. With today's complex paint formulations, the solvent mixtures can be a complex mixture of 8 to 12 different solvents that are designed to function with a particular paint solid. If the ratio of any of the components is substantially altered, the coating material will not necessarily perform in the desired manner. Therefore, a recovery system must not only effectively recover a wide range of solvents but must do so in the same ratio as originally formulated. Furthermore, an economical problem can be encountered when attempting to recover solvents in a stream of gas when the flow rate of the gas stream is less than about 5000 scfm. At such low flow rates, certain economies of size which make some solvent recovery systems efficient at high flow rates are not available. Accordingly, at these flow rates many prior art solvent recovery systems are simply economically inapplicable.